In a typical paging system, a broadcast station transmits message data, using radio waves, to the radio receivers of remote pagers. Most remote pagers are small portable devices carried by individuals. As portable devices, remote pagers carry a battery power source requiring replacement or recharge. It is, therefore, desirable that remote pager radio receivers be power efficient.
In U.S. Pat. No. 4,713,808, issued Dec. 15, 1987 to Gaskill, et al., remote pager power consumption is greatly reduced by a synchronized transmission protocol wherein radio receivers of individual remote pagers are active during predetermined associated time slots to monitor a selected one of several broadcast stations, or frequencies, for message data. The one or several broadcast stations are programmed to broadcast message data for a selected remote pager during time slots associated with that remote pager. Remote pagers, having radio receivers active only during brief monitor intervals, conserve power, yet reliably receive message data.
Synchronized transmission protocols, such as the above described Gaskill system, necessarily use a monitor interval longer than the associated time slot. Unavoidable timing errors preclude absolute timing precision. A longer monitor interval increases the possibility that the radio receiver will be turned on during the associated time slot. While the radio receiver is turned on, it captures radio transmitted message data. The pager then examines the captured data to locate pertinent message data, that is, data broadcast during the time slot associated with the pager. Because the monitor interval is necessarily longer than the time slot, the radio receiver is on for a time before the associated time slot and for a time following the associated time slot. Such additional radio receiver on time contributes to inefficient power consumption. Accordingly, it is desirable that the monitor interval be long enough to reliably receive message data, yet short enough to improve power conservation.
In any synchronized transmission protocol involving remote receiving devices, such as remote pagers, absolute synchronization is virtually impossible. Because the operating environment for each remote pager changes, the timing circuitry of each remote pager is affected and its timing or synchronization capability degraded. If the operating environment for the remote pager were absolutely static, and perfect timing circuitry were available, it would be possible for the remote pager to activate its radio receiver at the instant before an associated time slot. The radio receiver would be on substantially only during the associated time slot and radio receiver power consumption would be minimized.
Unfortunately, such perfection is not possible in the real world and it has been necessary to provide a monitor interval longer than the associated time slot. More particularly, to maintain high message reliability, it is necessary to provide a monitor interval long enough to accommodate a broad range of potential variations in environmental conditions through the lifetime of the remote pager. At one extreme of this range, environmental conditions place time slots at the beginning of the associated monitor interval, and at the opposite extreme, prevailing conditions place time slots at the end of the corresponding monitor interval. It is desired that time slots be generally centered within the corresponding monitor interval such that positive or negative timing errors do not affect message reliability. Thus, while it may be desirable to reduce the duration of monitor intervals to improve power conservation, such reduction can degrade message reliability because time slots are less likely to be centered relative to, or even concurrent with, the corresponding monitor interval.